Last month we showed you how to use resistors to reduce voltage. While it's a viable solution for many cases it isn't for others. As noted before, resistors work for devices whose load remains relatively constant: lights and some motors for example.
Where resistors fall short is with devices whose load varies: radios, gauges, and many electronic items. For those we're going to employ the components that make things like portable radios and computers possible: transistors.
When viewed from the printed side, most 78-series transistors match the following pin-out:
What follows are ways to use three common variants of the transistor. Two are voltage regulators, basically infant versions of the ones in modern alternators. One limits voltage to a fixed output; another requires a touch more preparation and can reduce your car's voltage to any figure as little as 1.25 V. The third transistor is a type of amplifier that boosts any of those regulators' current capacity.
This may sound intimidating if you've never worked with electronics but let us reassure you that what we're about to do is exceedingly easy-the simplest example requires only three wires. Just as the case last month, you can even copy our examples. If there's anything tricky about them it's that they require soldered connections. We won't go there, as that's general information already covered in countless books and online tutorials and videos.
The 78-series transistor is hands-down the simplest in the regulator family. The two numbers following the number 78 designate the regulator's output: 05 for 5 V (great for USB ports), 06 for 6 V (great for gauges), and up from there in increments of 07, 08, 09, 10, 12, 15, 20, and even 24. Two exceptions violate this protocol: 7833 is 3.3 V (great for huge arrays of parallel-wired super-bright LEDs) and 7847 is 4.7 V.
This diagram represents how a 78-series transistor works in a system. Feed the vehicle's p
As their numbering implies, these regulators' values are fixed, meaning their output remains constant regardless of input voltage, provided the input exceeds the output by a few volts and remains less than 30 or so volts. In some cases these regulators can handle 1 1/2 amps, certainly enough to operate gauges singly and probably in multiples. As a bonus, these regulators shut down if overheated or short circuited.
Adjustable Voltage Regulators
The 78-series regulators are great but can't address any voltage outside their specified range. Regulators in the LM family (LM138, LM317, and LM338, to name a few) can be "tuned" to reduce voltage all the way down to 1.25. Depending on the part number, some can handle as many as 5 amps.
Naturally, a little complexity accompanies this versatility. Two resistors govern the regulators' output. Here's their formula:
V = 1.25(1 + [R2 ÷ R1])
In most cases 78-series regulators can be wired directly. They do, however, require a heat
The value of any single resistor isn't important but the quotient of the two is. If we set the first resistor's value at 220 ohms and the second at 1,000 ohms the transistor regulates our input voltage to 6.93 V (theoretically, at least; as explained last month resistor values are usually within a range of their specified resistance but not exact). Here's how it looks on paper (remember to start inside the parentheses and work your way outward):
6.16 = 1.25(1 + [2,200 ÷ 560])
Here's the formula again, this time with a 1,000-ohm resistor for R1 and an 82-ohm resistor for R2.
1.35 = 1.25(1 + [82 ÷ 1,000])
Devout traditionalists will understand the significance of that last number: it's the same voltage as the obsolete battery in a Sun tachometer sending unit. Such a voltage reducer lets the sending unit wire directly to the vehicle's electrical system thereby eliminating the pesky battery altogether. And it's small enough to fit in the old battery socket. See the graph elsewhere in this story for other resistor combinations.
Take special note: the adjustable regulator's pin out doesn't match that of the fixed-outp
The hardest thing about an adjustable regulator isn't the regulator or the resistors; it's
A 78-series transistor's case or heat sink can touch the vehicle chassis but not so with t
Do yourself a favor and mount complex circuits to printed circuit board and cut them out.
We apologize up front for the next part as it's a bit confusing at first. Schematics for adjustable regulators flip the pins around. But it's not random; it's actually the simplest way to show what goes where. Like the fixed-regulator schematic, it reads from left (power in) to right (power out) and from top (positive) to bottom (negative). Here's how it looks:
Regulators typically handle 1 to 5 amps. But some applications, especially heater fans and old tube radios, can draw more than even the most powerful regulator can handle. Technically, you could combine the outputs from several regulators but the following shows a more elegant way.
The following transistor complements the regulator's voltage output, only with a lot more current capacity to back it up. Even the inexpensive 2N3055 from Radio Shack can handle 15 amps. That's enough to run a full deck of gauges and a blower motor at full speed and a tube radio at full song with power to spare. Need more output? One regulator can feed many complementary transistors for amps aplenty.
Like the others, the complementary transistor has three legs but only two have dedicated c
For simplicity's sake we haven't used the technically correct terms for transistor connection legs but they're sort of required to explain the complementary transistor's operation. The correct term for a transistor's input is the collector. The correct term for the transistor's output is the emitter. Transistors have what's called a base leg, only until now we referred to them as ground or adjust.
These base legs serve as a sort of reference for the regulator, only instead of referencing ground, the base leg in a complementary transistor references the voltage output of the regulator's emitter (output). In other words, instead of driving a device directly with its emitter (output) leg, the regulator drives the base leg of the complementary transistor.
Complementary transistors can mount as close to or as far away from the regulator. That's a good thing as these transistors usually require fairly big heat sinks; their capacity means they can generate lots of heat if asked to drive great current loads (fans and radios, for example). So long as their collectors (inputs) connect to the battery (preferably through an ignition switch), their emitters (outputs) connect to the devices, and their bases connect to the regulator's outputs (emitters), these complementary transistors can operate just about anywhere in a vehicle.
Here's what the symbol for a complementary transistor looks like. We retained the technica
The only potential issue with complementary transistors is that they don't reproduce the regulator's output exactly; they "eat" about 0.7 V. So if you connect a 6V regulator to a complementary transistor, it will emit 5.3 V.
If using an adjustable regulator, tune it to emit roughly 0.7 or more additional V. But remember that we said that you can't alter a fixed-voltage regulator's output? Well we lied; resistance in a fixed regulator's base connection (ground) will fool it into increasing its voltage output. A diode usually reduces output by 0.7 V, so routing a fixed regulator's ground through one will increase its output by the same amount. Slick, eh?
Common Resistor Values And Regulator Voltages
The formula to calculate resistor values to achieve a particular voltage is easy:
V = 1.25(1 + [R2 ÷ R1])
That said, not everybody wants to go through the motions. The following values will get you close; however, with one exception, you'll have to calculate your own values to offset the voltage loss when using a complementary resistor:
||Sun tachometer sending unit power
||Most red, orange, yellow, and green LEDs
||Most blue and white LEDs (but verify first)
||USB-powered electronics (MP3 players, phones, etc.)
||6V components (gauges, fans, and radios)
||6V components when using a complementary transistor
A fixed-output regulator circuit with a complementary transistor isn't necessarily more co
Here's a complementary transistor in a circuit with an adjustable regulator. The voltage-d
Complementary transistors require considerably larger heat sinks than regulators do. Old c
L-Com's handy panel-mount USB ports are inspiration enough to learn about transistors. The
For the record, ammeters can't benefit from voltage regulation as they're not voltage sens